The present invention relates in general to induction machines and in particular to rotating induction generators and motors.
Induction machine refers to a broad class of electromagnetic machines where force or rotational torque or electrical energy are produced by the interaction of a driven and a generated magnetic field and currents which occur because of induction. A rotating induction motor or generator is one of these induction machines. To create a rotating induction motor a cylindrically shaped stator or stationary element has stator coils that are disposed into slots in a magnetic material. A cylindrical rotor or rotating magnetic material element is disposed on the same center line as the stator and mechanically rotates on bearings placed outside the stator. The rotor typically has slots parallel to its center line and on the outside surface of the rotor into which conducting (typically copper) bars, which are electrically isolated from the rotor magnetic material, are placed. These conductor bars are then electrically connected on both ends of the rotor creating a number of loops of conductors wherein the axis perpendicular to the plane formed by each loop is perpendicular to the axial center line of the cylindrical rotor. Magnetic flux that couples through each of these conducting loops of the rotor may induce (by magnetic induction) currents to flow in the conductors. The interaction of the current in the rotor conductor loops and a rotating magnetic field is used to either create a motor (rotational torque producing machine) or a generator (alternating current producing machine).
Induction machines employing multiple phased electrical excitation may easily produce a rotating magnetic field by judicious placement of the stator coils around the cylindrical stator and proper connection of the coils so that the direction of the current flow in the coils creates magnetic fields that interact with corresponding induced currents in the rotor conducting coils.
When an induction machine is operated as a motor, then an energy source (e.g., a three-phase power line) is connected to the stator coils and the current that flows in the stator coils produces a magnetic field that in turn couples with corresponding rotor conductor loops which also produce a magnetic field. The stator field rotates because of the fact that the stator coils are driven by voltages with a shifted phase relationship (typically, 120 electrical degrees apart). The rotating magnetic field will create a torque on currents induced in the rotor conductor loops and the rotor will begin to rotate. Since the rotational speed of the magnetic field is dictated by the frequency of the energy source, the rotor rotation speed will lag that of the magnetic field of the stator. Depending on the torque load on the rotor shaft and the magnitude of the stator rotating field and the induced currents in the rotor, the rotor accelerates up to a rotation speed that is close to but slightly (a few percent) slower or higher than the rotation speed of the magnetic field. In the case of the motor, the excitation for the field of the rotor is provided by induction from the magnetic field of the stator coils. Multiple phased induction motors other than three phases are possible by the proper choice of the stator and rotor coils and the placement in the stator and rotor magnetic structures respectively. A single phase induction motor is also possible even though a single phase stator does not produce a rotating magnetic field. In the single phase induction motor the stator windings and the field windings are placed so that their axes are orthogonal (90 degrees).
Induction generators require some energy source to provide excitation of the field windings. This excitation along with providing mechanical rotation of the rotor conducting loops, enable energy stored in the field of the rotor windings to be transferred to an output or energy winding. In applications where the AC power grid of the power company provides the excitation of the generator, an engine may be used to rotate the rotor above a synchronous speed to allow the generator to both supply current to a load and also supply energy back to the grid. However, if a standalone operation is desired, then some other source of excitation must be supplied in case the power grid is disconnected or fails.
To run an induction generator in a stand-alone mode requires that another excitation source. This may be done by providing a field winding separate from an energy or output winding and driving the field windings with the separate excitation source wherein the frequency of the energy supplied to the output is governed by the rotational speed of the rotor. If auxiliary field windings with corresponding energy storage capacitance are added to the stator, then a self-excited induction generator is also possible. Residual magnetism in the rotor is usually adequate to start a process of self-excitation. The residual magnetic field, by rotating the rotor will excite by induction the auxiliary winding with the corresponding capacitance and cause the capacitor to be charged to a voltage which sometime later creates a current in the winding inductance. This current, in turn, induces a current in the rotor loops which again excites another auxiliary winding until the fields in the auxiliary windings reach a steady state with an excitation frequency dependent of the rotational speed of the rotor. To obtain optimal efficiency, there is a phase relationship between a field current and the corresponding output or stator current. However, this phase relationship may be influenced by the load current supplied by the stator winding receiving stored energy from the rotor. To keep a self-excited induction generator operating efficiently, the currents in the auxiliary field winding may have to be adjusted to generate a stable output with a variable load.
Producing a stand-alone induction generator which may supply a load when driven from a prime mover (e.g., engine), an induction generator operable to power condition energy from the AC power grid, and an induction system where energy may be supplied to the AC power grid, a variable load, or both, is desirable because of the low cost and simplicity of the induction machine. There have been many attempts to control the output of a self-excited induction generator, but no commercially viable system is on the market. There is therefore a need for an induction machine design where all the induction machine combinations are possible while allowing the induction machine to be controlled to produce energy efficiently with a low distortion and stable output voltage.
An induction generator which may have various configurations has one or more energy windings. The induction generators have one or more energy windings which may be electrically and magnetically isolated from each other by their relative positions as they are wound on the stator. Additionally, auxiliary windings are placed on the stator so that they are electrically isolated and either magnetically isolated from or coupled to the energy windings. The auxiliary windings have a no load capacitor placed across (in parallel) with individual coils of the auxiliary winding. Each winding also has a capacitor which may be selectively coupled (in parallel) with an individual coil by an electronic AC switch in series with the capacitor by gating the electronic AC switch with a load control signal. Each load control signal is generated in response to feedback signals comprising parameters of the voltage across each capacitor, an energy winding load voltage and the corresponding load current. Induction systems are constructed using variations of the induction generator to produce a stand-alone induction generator driven by a prime mover (e.g., engine) to supply an isolated load, an induction generator wherein the field is supplied by an AC power grid and the induction generator supplies power to the AC grid and/or an isolated load. Also a combination induction generator system is produced where the induction system receives energy from or supplies energy to the AC grid on a first energy winding and supplies an isolated load on a second energy winding wherein the efficiency, losses, and output voltage are regulated by feedback signals used to control of the capacitor combinations coupled to the auxiliary windings within the induction generator.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.